This application claims the benefit of Korean Patent Application No. 1999-50992, filed on Nov. 17, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to manufacturing liquid crystal display (LCD) devices, and more particularly, to a method and apparatus for injecting liquid crystal materials into liquid crystal display panels.
2. Discussion of the Related Art
A typical liquid crystal display (LCD) panel has upper and lower substrates and an interposed liquid crystal layer. The upper substrate usually includes common electrodes, while the lower substrate includes switching elements, such as thin film transistors (TFTs), and pixel electrodes.
As the present invention relates to manufacturing liquid crystal display panels, a brief explanation about conventional liquid crystal display manufacturing processes will be helpful. Common electrodes and pixel electrodes are formed on upper and lower substrates, respectively. A seal is then formed on the lower substrate. The upper and lower substrates are then bonded together using the seal such that the common electrodes of the upper substrate and the pixel electrodes of the lower substrate are opposed to each other, and such that liquid crystal cells are formed. Liquid crystal material is then injected into those cells through injection holes. The injection holes are then sealed. Finally, polarizing films are attached to the outer surfaces of the upper and lower substrates.
In operation, the light passing through the liquid crystal cells is controlled by electric fields that are applied via the pixel and common electrodes. By controlling the electric fields desired characters or images can be displayed.
While fabricating the various components of a liquid crystal display, such as the thin film transistors or the color filters, typically require numerous manufacturing steps, the overall fabrication process is relatively straightforward. FIG. 1 illustrates a typical liquid crystal panel manufacturing process in some detail. An initial step, st1, is to form an array matrix of thin film transistors and pixel electrodes over an array (lower) substrate.
The next step, st2, is to form an orientation film over the lower substrate. This involves uniformly depositing a polymer thin film over the lower substrate and then uniformly rubbing the polymer thin film with a fabric. The rubbing process involves rubbing the surface of the polymer thin film so as to orientate the film. A typical orientation film is an organic thin film such as a polyamide thin film.
The third step, st3, is to produce a seal pattern on the lower substrate. When the upper and lower substrates are attached, the seal patterns form cell spaces that will receive the liquid crystal material. The seal pattern will also prevents the interposed liquid crystal material from leaking out of the completed liquid crystal cell. A thermosetting plastic and a screen-print technology are conventionally used to fabricate the seal pattern.
The fourth step, st4, is to spray spacers over the lower substrate. The spacers have a definite size and act to maintain a precise and uniform space between the upper and the lower substrates. Accordingly, the spacers are placed with a uniform density on the lower substrate using either a wet spray method, in which case the spacers are mixed in an alcohol and then sprayed, or a dry spray method in which only the spacers are sprayed. The dry spray method itself is divided into a static electric spray method that uses static electricity and into a non-electric spray method that uses gas pressure. Since static electricity can be harmful to the liquid crystal, the non-electric spray method is widely used.
The next step, st5, is to aligned and attach the upper and lower substrates together, and to attach color filters to the upper substrate and the lower substrate. The aligning margin, which is less than a few micrometers, is important. If the upper and lower substrates are aligned and attached beyond the aligning margin, light leaks away such that the liquid crystal cell cannot adequately performed its function.
In the sixth step, st6, the liquid crystal element fabricated through the first five steps is cut into individual liquid crystal cells. Conventionally, a liquid crystal material was injected into the space between the upper and the lower substrates before the cutting into individual liquid crystal cells. But, as displays become larger, the liquid crystal cells are usually cut first and then the liquid crystal material is injected. The process of cutting typically includes scribing using a diamond pen to form cutting lines on a substrate, and a breaking step that separates the substrate along the scribed lines.
The seventh step, st7, is to actually inject liquid crystal material into the individual liquid crystal cells. Since each individual liquid crystal cell is a few square centimeters in area, but has only a few micrometer gap between plates, a vacuum injection method is effectively and widely used. Generally, injecting the liquid crystal material into the cells takes the longest manufacturing time. Thus, for manufacturing efficiency, it is important to have optimum conditions for vacuum injection.
FIG. 2 shows a conventional vacuum injection process for injecting liquid crystal material into a liquid crystal cell. To inject the liquid crystal material, a liquid crystal cell 2 having an injection hole 4 is placed inside a vacuum apparatus 6. The liquid crystal cell is located over a vessel 8 that contains the liquid crystal material 10. During operation, suction removes air from the vacuum apparatus 6 to create a strong vacuum.
In practice it is possible for small air bubbles in the liquid crystal material 10 to gradually add together to form a larger air bubble. Such air bubbles can cause problems. Accordingly, before injection, the liquid crystal material should be left under a vacuum of a few mTorr for a sufficient time that the air bubbles in the liquid crystal material 10 are removed. Conventionally, the vessel 6 containing the liquid crystal material 10 and the liquid crystal cell 2 are all left under this vacuum condition.
One method of injecting the liquid crystal material into a liquid crystal cell is to dip the liquid crystal cell into the tray containing the liquid crystal material. However, the dipping method consumes too much of the liquid crystal material. Another method involves touching (slightly dipping) only the injection hole 4 to the liquid crystal material. Still referring to FIG. 2, in the touch method, after air in the liquid crystal cell 2 and in the liquid crystal material 10 has been removed, the injection hole 4 is slightly dipped into the vessel 8 containing the liquid crystal material 10. At first, the liquid crystal material 10 is injected into the liquid crystal cell 2 by capillary forces. Later, nitrogen gas is introduced into the vacuum apparatus 6. The difference in pressure between the interior and exterior of the liquid crystal cell 2 forces liquid crystal material 10 into the liquid crystal cell 2.
FIG. 3 is a graph illustrating the pressure in the vacuum apparatus 2 with respect to time. During period xe2x80x9cAxe2x80x9d, a vacuum condition is being formed. At the end of period A the injection hole 4 is dipped into the vessel 8 containing the liquid crystal material 10. During period xe2x80x9cBxe2x80x9d, the liquid crystal molecules are pressure injected into the liquid crystal cell.
After injection of the liquid crystal material is complete, the injection hole 4 is sealed with an epoxy-based sealant that is applied through a dispenser.
FIG. 4 illustrates a method of measuring the viscosity of the liquid crystal. A liquid crystal material having one of the orientations xe2x80x9cn1xe2x80x9d, xe2x80x9cn2xe2x80x9d, or xe2x80x9cn3xe2x80x9d is interposed between a fixed substrate 15 and a movable substrate 20. The movable substrate 20 is then moved parallel to the fixed substrate 15. As the movable substrate 20 moves, it produces a shear stress due to the viscosity of the liquid crystal.
As shown in FIG. 5, a liquid crystal material oriented in the direction xe2x80x9cn1xe2x80x9d has the highest viscosity since xe2x80x9cn1xe2x80x9d orientated liquid crystal molecules produce the greatest shear stress. However, liquid crystal molecules orientated in the direction xe2x80x9cn2xe2x80x9d have the lowest viscosity. Furthermore, liquid crystal molecules orientated in direction xe2x80x9cn3xe2x80x9d have an intermediate viscosity.
Conventionally, liquid crystal material is injected into a liquid crystal cell without regard to the orientation of the liquid crystal molecules. FIG. 6 shows liquid crystal molecules 50 being injected into the gap between vertically orientated orientation films 44 on the lower and the upper substrates 30 and 40 by way of a conventional vertical alignment (VA). As liquid crystal molecules 50 move in the direction of the arrow, they tend to become aligned vertically by the vertically orientated orientation films 44. That is to say, since liquid crystal molecules having the orientation xe2x80x9cn1xe2x80x9d are more latched by the molecules of the orientation film 44 than liquid crystal molecules having orientations xe2x80x9cn2xe2x80x9d or xe2x80x9cn3,xe2x80x9d the xe2x80x9cn1xe2x80x9d liquid crystal molecules gather on the surface of the vertical orientation film 44 and align vertically. The liquid crystal molecules adjacent to the vertically aligned liquid crystal molecules also tend to align vertically due to a mutual interaction. As the liquid crystal molecules become more remote from the orientation film 44, the liquid crystal molecules tend to become xe2x80x9cn3xe2x80x9d orientated.
Since in the conventional apparatus the orientations of injected liquid crystal molecules are mainly xe2x80x9cn1xe2x80x9d and xe2x80x9cn3xe2x80x9d, the injected liquid crystal material tends to have a relatively high viscosity, and thus the injection of the liquid crystal material is relatively slow.
One method of increasing the rate of injection is to inject at relatively high temperatures. However, this tends to deteriorate the liquid crystal material. Therefore, an improved method and apparatus for injecting liquid crystal material into a liquid crystal cell would be beneficial.
Accordingly, the present invention is directed to a liquid crystal injection apparatus and method that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to increase the injection rate of a liquid crystal material into a liquid crystal cell.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
The present invention provides for a liquid crystal injection apparatus for injecting a liquid crystal material into a gap between first and second substrates. The liquid crystal injection apparatus includes a first and a second electrodes that are electrically connected to an electric source that applies a potential to the first and second electrodes such that an electric field is created between the first and the second substrates, and such that the rate of liquid crystal material injection is enhanced.
Further, the present invention provides a method of injecting a liquid crystal material into a cell. The method includes preparing first and second substrates, each substrate having an orientation film; attaching the two substrates such that the orientation films of the substrates oppose each other; applying an electric field through outer surfaces of the first and the second substrates; and injecting a liquid crystal into a space between the first and the second substrates.
The present invention further provides a method of injecting a liquid crystal material into a liquid crystal cell defined by first and second substrates. The method includes the steps of applying an electric field through first and second substrates and then injecting a liquid crystal material into a gap between the first and the second substrates. Beneficially, the liquid crystal material is injected using either a capillary force or a pressure differential between the interior and the exterior of the liquid crystal cell.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.